Terahertz (THz) radiation, an electromagnetic radiation in a frequency interval from 0.1 THz to 10 THz, occupies the portion of the electromagnetic spectrum between the microwave band and the infrared band.
A THz photon has an energy that is less than the energy of an optical photon. That is why THz-ways can penetrate deep into the substance where the optical waves can not penetrate. At THz frequencies the molecules vibrate. That is why THz waves are useful in the study of molecules.
Indeed, the unique rotational and vibrational responses of molecules within the THz range provide information that is generally absent in optical, X-ray and NMR images. A THz wave can easily penetrate and inspect the insides of most dielectric materials, which are opaque to visible light and low contrast to X-rays, making THz waves a useful complementary imaging source.
For example, THz waves maintain reasonable penetration depth in certain common materials, such as clothes, plastic, wood, sand and soil. Therefore, THz technology has the potential to detect explosives packaged or buried within these materials because the explosives have unique THz spectral properties when compared to the surrounding materials. The spectral fingerprints of explosive materials can be expected in the THz band, and THz imaging can be applied for landmine detection.
However, at present, efficient, compact, solid-state sources for the spectral range 0.1-10 THz are still lacking.
Indeed, broadband pulsed THz sources are usually based on the excitation of different materials with ultra short laser pulses. A number of different mechanisms have been exploited to generate THz radiation, including photo carrier acceleration in photo conducting antennas, second-order non-linear effects in electro-optic crystals etc. Currently, conversion efficiencies in all of these sources are very low, and consequently, the beam powers are in the nanowatt to microwatt range, whereas the aver age power of the femtosecond optical source is of order of 1 W.
For narrowband THz sources, solid-state lasers are often considered. They are based on inter-band transitions in narrow gap semiconductors or on inter subband transitions, i.e. transitions in quantum confined structures, e.g. nanostructures, between confined conduction or valence states. To get THz radiation from direct inter band transitions, near zero gap semiconductors are required. For inter subband transitions conventional wide gap materials can be used, but require precise complicated structures. At present the construction of multiple quantum-well semiconductor structures for laser emission is feasible. The quantum cascade consists of a repeating structure, in which each repeat unit is made up of an injector and an active region. In the active region a population inversion exists and electron transition to a lower energy level occurs, emitting photons at a specific wavelength. Recently Kohler et al. (R. Kohler et al., Nature 417, 156 (2002)) designed a THz quantum cascade laser operating at 4.4 THz. The laser consisted of a total over 700 quantum wells, and demonstrated pulse operation at temperature of 10 K. For a review, please see, e.g., B Ferguson and X.-C. Zhang, Nat. Matter, 26 (2002).
Recently M. Dyakonov and M. S. Shur have proposed to use plasma wave electronics for THz applications. Please, see Phys. Rev. Lett. 71, 2465 (1993). They argued that a channel of a field effect transistor might act as a resonance cavity for the plasma waves. For micron or sub-micron gate length, the fundamental frequency of this cavity is in the THz range. Since electromagnetic radiation can excite plasma waves, such a device can be used for the resonance detection and mixing of electromagnetic radiation at THz frequencies. Under certain conditions the steady state with a dc current in the field effect transistor is unstable against spontaneous generation of plasma waves, which in his turn should lead to generation of electromagnetic radiation. The plasma instability can, however, be suppressed if there are appreciable losses at the contacts.
The stimulated Terahertz emission from inter-exitonic transitions in Cu20 was also observed. Please, see R. Hubes, B. Schmid, Y. Ron Shen, D. S. Chemla and R. A. Kindl, Phys. Rev. Lett., 96, 017402 (2006).